Energy source tracking system employing drift-line technique

ABSTRACT

A tracking system approach which, unlike that of systems employing closed loop tracking techniques, points the boresight axis of an antenna at a computed future position of a radiating energy source. The source is allowed to drift through the conical scanning beam, during which time-energy detection measurements are made on a periodic basis. A computer receives the energy measurements and computes therefrom the equation of the best straight line corresponding to the error versus time relationship, whereupon the slope and intercept of this fitted line are utilized to compute pointing error. The system is then periodically updated on the basis of this pointing error.

United States Patent 1 1 3,699,324 Iliff et a1. Oct. 17,1972

[54] ENERGY SOURCE TRACKING SYSTEM 3,571,479 3/1971 Horattas et al...343/5 DP EMPLOYING DRIFT-LINE TECHNIQUE 3,579,237 5/ 1971 Steingart etal ..343/5 DP [72] Inventors: Walter R. Iliff; John M. Holt; FOREIGNPATENTS OR APPLICATIONS George E. Chadima, all of Cedar Rapids; Robert GBrown Ames all 1,062,553 3/1967 Great Britain ..343/7.4

f l C II" R d C g s g 3: a lo ompany Primary Examiner-Felix D. GruberAttorney-Richard W. Anderson and Robert J. Craw- [22] Filed: Sept. 17,1970 f d 21 A l.N 72,910

[ 1 pp 57 ABSTRACT 52 U.S.Cl. ..23s/1s0.27,235/150.2s,343/117 A trackingSystem aPPmach which f 51 Int. Cl. ..G06f 15/50, GOls 7/46 SystemsemPbYing clsed "ackmg [58] Field of Search 235/150 27 150 2 150 25points the boresight axis of an antenna at a computed 23 343/7' 4 12 Zfuture position of a radiating energy source. The source is allowed todrift through the conical scanning [56] References Cited beam, duringwhich time-energy detection measurements are made on a periodic basis. Acomputer UNITED STATES PATENTS receives the energy measurements andcomputes therefrom the equation of the best straight line cor- 3l949497/1965 Jasperson "235/1502? responding to the error versus timerelationship, 3277468 10/1966 cflspers whereupon the slope and interceptof this fitted line 3'30455l 2/1967 Kikuo et "343/117 are utilized tocompute pointing error. The system is 3,305,868 2/1967 Kllsuo et"343/117 then periodically updated on the basis of this pointing3,316,552 4/1967 Reid ..343/117 erron 3,353,183 11/1967 Giger ..343/7.4X 3,396 5/1968 Van Popta et a1. ..343/5 DP 7 Claims, 7 Drawing Figures F22 INTGEQSTREEXDTORS DETECTION 26 BEARING f FlLl FglNG f SYNCH,CIRCUITRY DETECTOR MULT| F PLExER I r28 I 36 SWITCHING ELEVATION ISYNCH. ELEVATIO 39 r j DETECTOR NUTATION .1, GATE 30 I SEGMENT TIMINGRESET I COMMUTATOR GENERATOR 32 33 24 I T s NuTATIoN |9 38 AND BORESIGHTAXIS 2| POSITIONING ERG-EL POINT I8 COMMANDS en- COMPUTER DATA TOCOKMPUTER A/D I5 A6 |4 CONVERTER POsITI0N'--\ i F\ INERTIAL IN RORMEASUREMENT-I2 p0SITION UNIT PATENTEDncI 11 m2 3.699.324

/ PREDICTED SUN PATH b (OR MOON PATH) A (EPHEMERIS) 1* TIME b =A CHANNELPOINTING ERROR A CHANNEL OUTPUT AE=SYSTEM GAIN- m CORRECTION FACTOR m HW N- 5) I FIG.2

I RADIOMETRIC RECEIVER POINT q |4- COMMAND 4'-\ PBORSGTFOLNPOSITION+ERROR I3 VOLTAGE ERROR l2 r w INERTIAL MEASUREMENT L 'JQ-DEVICE POSITION ERROR nvvnvrons.

CLOCK WALTER R. ILIF'F FIG. 3 uomv M. HOLT GEORGE E. CHAD/MA ROBERT 6.BROWN BY 14 0M AGENT PA TENTEI] 17 3.699 324 SHEEI 3 0F 3 I IO 4 IREOEIVER |3 hi4 ASSUMED POINT ORBITAL COMMAND PARAMETER 8R6 EL PLusERROR POS'TION l7 VOLTAGE ERROR 5| f ANTENNA ORBITAL DIGITALGEOGRAPHICAL- PARAMETER POsITION uNIT COMPUTER 53 I 52 5o la ORBITALPARAMETER CLOCK ERROR F l G 5 TO RECVR. (FIG. 4) CLOCK DATA FROM BRG-ELl7 REcEIvE (FIe.4I

POINT COMMAND DIGITAL cOMPuTER ORTHOGONAL A -52 POINT ERROR BRG-ELASSUMED PARAMETER OR-B ITAL PAR AMETER FIG,6 PLUS ERROR PARAMETER ERRORUNIT 50 I KNOW GEOGRAPHICAL 53 ANTENNA LOCATION INPUT PARAMETER ERROR{S-CURVE LINEAR F l G .7 OPERATING PORTION\ T|ME INVENTORS.

WALTER R. ILIFF JOHN M. HOLT GEORGE E. CHAD/MA ROBERT 6. BROWN BY QMAGENT ENERGY SOURCE TRACKING SYSTEM EMPLOYING DRIFT-LINE TECHNIQUE Thisinvention relates generally to the tracking of energy radiating bodiesand more particularly to an improved passive tracking system foremployment with a receiving system of the type providing orthogonalpointing error signals. Application to tracking radiometric sextantswill be emphasized for purposes of exemplification and not by way oflimitation.

Known automatic passive tracking systems (radiometric sextants) aresimilar to radar tracking systems in that an antenna pattern is nutatedabout a boresight axis that extends from the antenna to the target orradiating source. Radiometric sextants include receiving circuits fordetecting energy that is radiated from a distant object and includephase detecting circuits that are synchronized with the nutation of theantenna about the boresight axis. These types of devices detect radiatedenergy in certain microwave bands from celestial bodies such as the sun,the moon, or particular stars, and the relative intensity of thedetected output of these certain microwave frequencies determines thedirection the antenna is to be positioned to point at the body that isbeing tracked. The radiometric sextant employs a nutating antennatracking system which is similar in many respects to tracking radars inthat a phase detection process is operated from the energy levelobtained from the conically scanned antenna beam which develops bearingand elevation pointing error signals to be employed in directing theboresight axis of the nutating antenna directly at the radiating body.

The present invention, while being described with respect to an improvedtracking system for a radiometric antenna, might equally be employed ina satellite tracking system or other type of tracking system the purposeof which is to track an energy radiating body for which the preciseorbital path is either known or desired. Thus the source of energyradiation might be any one of the planets the orbital paths of which aredefined in detail by ephemeris data, or might be a manmade satellite theprecise orbital path of which is either known or desired in terms oftime and position.

Generally the system of the present invention is applicable to trackinga radiating body from which level of energy radiation is to be measuredand which body follows an orbital path with respect to the earth.

Referring again to a radiometric sextant of the type utilized to track aradiating celestial body by utilizing the electromagnetic energyradiated by that body, a microwave radiometer function is employed todetect the thermal radiation received by a directional antenna and thusproduce an electrical output signal proportional to the receivedmicrowave energy. The precise direction of arrival of this energy isdetermined by conically scanning the antenna beam about a boresight axisextending in the general direction of the energy source. The phase andamplitude of the resulting output signal, as compared to a referencescanning rate signal, is interpretable as an angular vector between theconical scan axis and the true direction.

Known radiometric sextant tracking systems employ a closed loop trackingtechnique wherein the phase and amplitude of output signals are employedcontinuously to correct the direction of the scan axis to make itcoincide as accurately as possible with the direction of energy arrival.Thus the closed loop sextant tracks the source through a servo mechanismwhich seeks to minimize the amplitude modulation (seek the null) of theoutput signal from the sextant.

Systems of the closed loop type as concerns tracking are subject tovariable tracking error due to variations in internal gain, variationsin atmospheric transmittance and changes in thermal radiationintensities from celestial bodies. Further, solar intensities have rapidvariations during active periods, daily variations due to the slowlyvarying component of activity, and a very slow variation correspondingto an 11 year cycle of activity. Similarly lunar intensity varies withboth its phase of illumination and its distance from the earth (whichchanges its angular diameter).

The present invention has as a primary object thereof the developmentand incorporation into a tracking system of a drift-line trackingtechnique devised to eliminate the problem of total system gainvariations by providing a continuous measurement of system gain. Thusthe tracking system of the present invention is self-calibrating in thesense that angular pointing error determinations are essentiallyindependent of system gain variations, fluctuations in the power levelof the source, and transmittance of intervening media.

The present invention is featured in the provision of a synchronousdetection process which is referenced to antenna beam position in itsconical scan, wherein the radiometric sextant output modulation signalis split into quadrature components which correspond to elevation errorsignals and orthogonal line of sight bearing error signals respectively.The basic principle of the present invention, as defined by thedrift-line tracking technique, is that, with a stationary scan axis andsufficiently small pointing error, the error signal level in either theelevation or line of sight bearing channel is approximately a linearfunction of pointing error and, therefore, a linear function of time dueto the apparent angular velocity of the source imparted by earthrotation. Therefore, the ratio of the measured modulation rate of changeto the predicted source angular velocity is a direct measure of totalsystem gain in terms of modulation per unit of angular pointing error.

The tracking technique of the present invention is featured, anddistinctly unlike that of systems employing closed loop trackingtechniques, in the employment of a system wherein the boresight axis ofthe antenna is pointed at a computed future position of the radiatingenergy source, and the source is allowed to drift through the conicalscanning beam, during which time energy detection measurements are madeon a periodic basis. A digital computer receives the energy measurementsand determines the slope and intercept of a straight line fit to thetime-energy measurements. The slope and intercept of this fitted lineare utilized to compute pointing error. The system is then periodicallyupdated on the basis of this pointing error.

The drift-line technique to be described herein, effectively solves theproblem of potentially inaccurate conversion of output error signal toabsolute angular pointing error by eliminating the effects of unknown orill-defined system gain change. During each drift period, system gain ismeasured thus providing a determination of pointing error which isindependent of gain changes.

These and other objects and features of the present invention willbecome apparent upon reading the following description with reference tothe accompanying drawings in which:

FIG. 1 is a diagrammatic representation of the drift technique antennapointing geometry;

FIG. 2 is a typical representation of drift technique data as concernspointing data versus time;

FIG. 3 is a functional diagram of a drift technique pointing or trackingsystem useful for navigation in accordance with the present invention;

FIG. 4 is a more detailed functional diagram of a particularimplementation of the basic system illustrated in FIG. 3;

FIG. 5 is a function diagram of a drift technique tracking system fordetermination of source position and orbital path; and

FIG. 6 is a functional modification of the FIG. 4 system implementationfor use in source location; and

FIG. 7 represents a typical S curve of received energy versus time for aradiation body passing through the aperture of a fixed tracking antenna.

The tracking technique of the present invention, as depicted in FIG. 3,employs a radiometric sextant for detecting radiating energy from acelestial body or other radiating body of known orbital position, adigital computer, an inertial measurement unit, and a timing or clocksource.

The system of FIG. 3 is utilized to update the inertial measurementdevice based on calculated information obtained from the known positionof a radiating celestial body. Inertial measurement devices mayaccurately predict changes in longitude and latitude from an initialposition by means of various integration processes performed onaccelerometers, however, such units, depending upon their complexity,are subject to drift errors and must be periodically updated orcalibrated. The present invention then, as employed in the FIG. 3system, might be basically defined as a system providing an update on aperiodic basis for an inertial guidance system. This update is performedon the basis of celestial observations by means of the radiometricsextant or, as previously mentioned, might be performed on the basis ofreceiving energy from any body from which energy is caused to beradiated, which body is known to follow a precise orbital path which canbe defined by ephemeris data.

The drift-line technique might generally be defined as pointing theboresight of a nutating antenna at the predicated position that aradiating body will occupy a discrete time in the future. As theradiating body passes through the aperture of the fixed antenna, thereceived energy level should pass through the zero or crossover portionof the S curve of the received energy at time zero if the predictedfuture position is accurate. FIG. 7 depicts a typical S-curve of energyversus time obtained by a stationary antenna with the radiating bodypassing through the antenna aperture. If the predicted point at whichthe body is expected to be in the future is inaccurate, the S curve willbe displaced with respect to the zero axis, that is to say, at time zerothe received energy will not be passing through the zero axis of the Scurve of received energy. Means may then be employed to 6 define thelinear portion of the S curve in terms of the known definitions of astraight line, that is, the slope and axis intercept. As will be furtherdescribed, data may be taken from which a digital computer can determinethe slope and intercept of a straight line fit to a number of dataacquisition points over a prescribed interval of measurement. From theslope and intercept of the line fit to the received data and thefunction of the ephemeris data determined straight line for theradiating object, the computer may compute the pointing error of theboresight axis of the antenna in both elevation and bearing, and thesystem may utilize these pointing errors to correct the inertialmeasurement device.

In basic operation, the system depicted functionally in FIG. 3 may bedefined as follows:

1. The inertial measurement unit furnishes an assumed position of thetracking antenna to the digital computer-this assumed position beingequal to the true position plus any error which exists at the moment inthe inertial measurement unit.

2. The digital computer utilizes the assumed position input and timefrom the clock source to generate azimuth and elevation pointingcommands for application to the radiometric sextant antenna. Thecommanded position for the radiometric sextant antenna boresight axis isfor a future position of the radiating body, that is, for the positionthrough which the radiating source is expected to pass at intercept timea prescribed time interval from the present, for example 1 minute.

3. The radiometric sextant antenna boresight axis remains stationary fora predetermined time interval (for example, two minutes) during whichperiod the output is defined as error voltages in two coordinates versustime, the two coordinates being line-ofsight bearing and elevation.

4. The digital computer calculates the slope and intercept of a straightline fit to some length of data acquisition at a predetermined samplerate from the radiometric receiver, utilizing, for example, the leastsquares method. The slope of the fitted line and the angular velocity ofthe radiating body yield the angular equivalent of error voltage. Thefitted line intercept yields angular pointing error from AV/At (thesextant output) and from the constant relationship of AO/At (ephemeris)the computer derives AO/AV as the intercept conversion.

5. The computer translates the pointing error into error in the assumedposition which the inertial measurement unit initially furnished, and

6. The inertial measurement unit assumed position (which included thetrue position plus error) is gradually updated or corrected by thecomputer output.

7. A repeat cycle is initiated at which time the antenna boresight axisis again positioned at a point through which the radiating body isassumed to pass in one minute and the antenna again remains stationarywhile the radiating body passes through the antenna aperture, yieldingfurther data acquisition in bearing and elevation from the radiometricsextant output channels.

The drift technique antenna boresight pointing geometry is depicted inFIG. 1 which illustrates the boresight location which is held for twominutes. The sun or moon or other radiating celestial body is initiallyone minute behind the boresight location and, at the conclusion of thecomputation cycle, is one minute ahead of the boresight location.

The drift technique data principle is illustrated graphically in FIG. 2which shows a plot of error versus time-the intersection of the errorand the time axes being the intercept at time zero. The predictedradiating body path is depicted as a solid line having a zero axisintercept and slope defined as m The dashed line depicts the line fittedby the digital computer based on the received energy level samplings forthe time preceding and following the zero axis intercept time. Thedashed line is indicated as having a channel pointing error b and aslope m it being understood that FIG. 2 depicts the situation for butone of the two axes-for example, the elevation axis and depicts dataacquisition within the linear portion of the receiver S curve (FIG. 7).The system gain correction factor is constantly obtainable by computingthe ratio of the slopes of the predicted path and the actual path ascalculated by the computer, such that the corrected pointing error forthe channel may be defined as the channel pointing error multiplied bythe ratio of the slopes of the two lines. Again it is to be understoodthat the process and technique would be repeated for the orthogonal(line-of-sight) bearing channel, and the detection technique employed inthe radiometric receiver provides means for developing first and secondoutputs respectively indicative of bearing and elevation erros.

By way of emphasis and summary the drift-line technique outlined hereinis basically implemented as follows:

1. The radiometric sextant antenna is commanded to point in thedirection of the source computed for some predetermined length of timeafter initiation of the command, based upon assumed sextant position andthe radiating energy source ephemeris data.

2. For a predetermined drift period the source moves linearly throughthe region conically scanned by the antenna pattern about a stationaryscan axis.

3. During the drift period, the elevation and line-ofsight bearingchannel outputs are sampled at a predetermined periodic rate,digitalized, and fed to a digital computer by the radiometric sextant.

4. The computer determines the slope and intercept of a straight linefit to the sampled outputs during the drift period.

5. The slope and intercept of the fitted line are used to compute thepointing error which is further translated into error in assumedposition for subsequent updating of the inertial measurement unit fromwhich the initial assumed position was taken in step 1.

FIG. 4 illustrates a functional embodiment of a driftline system inaccordance with the present invention by means of which the detectedenergy levels corresponding to the bearing and elevation components ofthe received energy are multiplexed in a time-sharing arrangement andapplied (after analog to digital conversion) to the digital computer asdata input.

Received energy from the antenna member 10 is supplied to a detectionand filtering system 22 which might comprise the front-end of aradiometric receiver. The detection circuitry 22 provides an output 26in the form of a voltage level corresponding to the received energy. Itis to be understood that this voltage level is a continuously varyingfunction when the antenna boresight axis is not aligned directly on theenergy radiating body. Through a mechanical linkage 25 with the antenna10, a nutation commutator 23 is driven to provide outputs 39 and 40which correspond respectively to azimuth and elevation phase-positionreferences as concerns the alignment of the boresight axis with theradiating body. Reference is made to US. Pat. No. 2,969,540 to ClydeStewart, assigned to the assignee of the present invention, for adescription of antenna nutation commutation arrangements which areemployed in tracking antenna devices such as a radiometric receiver.

In the embodiment of FIG. 4, the outputs from the nutation commutator 23are applied to a gate timing generator circuitry 41 which, under thecontrol of the input 20 from master clock 19, generates appropriatetiming and gating outputs. Gate outputs 29 and 30 are utilized ascontrol inputs to synchronous detectors 27 and 28 such that the bearingsynchronous detector 27 provides an analog output corresponding toenergy level variations in a first axis while the altitude synchronousdetector 28 provides an output in analog form corresponding to theenergy variation in a second axis. The output from the receiverdetection and filtering circuitry 22 is applied as inputs to each of thesynchronous bearing detectors. In operation, the synchronous bearingdetectors separate those portions of the output 26 from the detector 22into line separated voltage levels corresponding respectively to bearingand altitude components of energy level measurement. The outputs fromthe synchronous detectors are applied to a pair of gated integrators 35and 36 which store signals corresponding to the energy levels over acalculation period and apply the signals to a multiplexed switchingarrangement 37. The gated integrators 35, 36 are controlled by resetcontrol circuitry 33 under the influence of reset timing pulses 31 fromthe gate timing generator 41 which are in turn timed by the nutationcycle time. The multiplexed switching arrangement 37 is controlled by asampling output 32 from the gate timing generator 41 such that theoutput 38 from the multiplexed switching circuitry 37 corresponds tosequential or multiplexed error signals corresponding to bearing andaltitude. These signals, analog in nature, are applied to ananalog-to-digital converter 42 which under the control of clock signals21, develops appropriate digital data corresponding respectively toaltitude and azimuth error signals in the form of bits per unit of timeas an e input 14 to digital computer The sampling effected by themultiplexer switch arrangement 37 is at a predetermined rate, forexample, every one-tenth second. This sampling is continuously effectedduring the two minute period of time during which the boresight axis ofthe antenna is fixed and the radiating body passes through the apertureof the antenna. At the conclusion of a nutation cycle the reset 31 fromthe gate timing generator 41 sets the gated integrators 35 and 36 tozero to ready them for an ensuing calculation period.

Digital computer 17 utilizes the data input 14 from theanalog-to-digital converter to calculate the intercept and slope of astraight line fit to the data in each of two axes and, as previouslydiscussed, develops bearing and elevation pointing signals 13 as commandsignals for application to the antenna mutation and bore axispositioning mechanism 24. Digital computer 17 applies the previouslydiscussed error in position updating signal 15 to the inertialmeasurement unit 12, while the inertial measurement unit 12 provides aposition plus error input 16 to the digital computer.

The present invention in the above-described embodiment thus provides atracking technique employing direct measurements of atmospheric andradiometric variations for use in correcting pointing error data, andplaces the responsibility of pointing accuracy more directly upon theephemeris data used to point the sextant to the radiating source.

The above described embodiment utilizes the driftline technique of thepresent invention as an improved tracking means in conjunction with aninertial navigation system for the purpose of updating the inertialnavigation system. The source being tracked is defined by knownephemeris data used as reference.

The technique of the present invention is additionally applicable tooptical star tracking systems or radio tracking systems for radiationsources where radiation source location parameters are to be determined.

The drift-line technique, as applied to determination of orbitalparameters of an energy radiating source, is depicted functionally inFIG. 5 wherein the orthogonal bearing and elevation position voltageerrors from the receiver 11 are again applied to the digital computer17. As in the previously described embodiment, the computer providespointing commands 13 to the antenna system associated with the receiverand, generally speaking, the computer is programmed similarly to theprevious embodiment with the exception that the antenna of the receivingsystem is considered to be located at a reference position defined byknown geographical position parameters. The basic system of tracking asdepicted in FIG. 5 includes an orbital parameter readout unit 50 whichprovides an input on line 51 to the digital computer in the form ofassumed orbital parameters of a source being tracked plus some error.The computer is programmed to receive this information, in conjunctionwith the bearing-elevation position voltage errors from the receiver,and to develop an output 52 applied back to the orbital parameter unitwhich corresponds to orbital parameter error per se for updatingpurposes. Thus, rather than utilizing the drift-line technique to updatea navigation system, the embodiment of FIG. 5 utilizes the drift-linetechnique to update the orbital parameters of a radiating source forwhich orbit determination is desired.

In basic operation then the tracking embodiment depicted functionally inFIG. 5 is defined as follows:

1. The position of the observer (that is the receiver known geographicalposition) is furnished from independent navigation sources and thisknown position is furnished to the digital computer as a reference inputparameter.

2. The digital computer utilizes assumed orbital parameters of thesource being tracked and time from the clock source to generate bearingand elevation pointing commands for application to the receiving systemantenna. The commanded position for the antenna boresight axis is for afuture position of the radiating body, that is, for the position throughwhich the radiating source is expected to pass at intercept time at aprescribed time interval from the present.

3. The receiving system antenna boresight axis remains stationary forapproximately twice the prescribed time interval above during whichperiod the output is defined as error voltages in two coordinates versustime, the two coordinates being line-of-sight bearing and elevation.

4. The digital computer determines the slope and intercept of a straightline fit to some length of data acquisition at a predetermined samplerate from the receiving system. The slope of the line and the angularvelocity of the radiating body yields the angular equivalent of errorvoltage.

5. The computer translates the pointing error into error in the assumedorbital parameters of the radiating source being tracked, and

6. The orbital parameters originally inserted which included the assumedorbital parameters plus error) are updated or corrected by the computeroutput.

7. A repeat cycle is initiated during which the antenna boresight axisis again positioned at a point through which the body being tracked isassumed to pass in the predetermined period of time and a further dataacquisition is obtained in bearing and elevation from the receiveroutput channels.

FIG. 6 represents a functional modification of the inertial measurementunit updating embodiment of FIG. 4 for operation as a system fordetermination of radiating source orbital parameters. With reference toFIG. 6, the data from the receiver of FIG. 4, in the form of orthogonalpointing errors in bearing and elevation, is applied on line 14 to thedigital computer 17 as in the previous embodiment. The digital computerreceives an assumed orbital parameter (plus error) input from theorbital parameter unit 50 on line 51 and, through the above outlinedprogram, develops source orbital parameter error output on line 52 as anupdating input to the orbital parameter unit 50.

Although the present invention has been described with respect toparticular embodiments thereof, it is not to be so limited as changesmight be made therein which fall within the scope of the invention asdefined in the appended claims.

We claim:

1. A navigation system comprising a receiver of energy from an energyradiating source having known orbital parameters, a computer means, aninertial measurement means, said receiver including radiation sensingmeans and means for nutating said sensing means about a steerableboresight axis, said receiver developing first and second outputvoltages respectively indicative of instantaneous bearing and elevationaxis pointing errors between the boresight axis of said sensing meansand said energy radiating source, said inertial measurement meansdeveloping output signals defining the earth geographical position ofsaid radiating sensing means, means for feeding said inertialmeasurement unit output signals as a first input parameter to saidcomputer, said computer receiving ephemeris data defining thetime-position of said energy radiating source as a second inputparameter thereto, said computer being programmed to calculate from saidfirst and second input parameters, bearing and elevation pointingcommands for application to said radiation sensing means based on acomputed future position of said source of energy radiation apredetermined time interval from the instant of said calculation, saidcomputer being programmed to effect an ensuing calculation period duringwhich zero pointing command is applied to said radiation sensing meansand during which time said energy radiating source passes through theaperture of said radiation sensing means, said calculation periodcorresponding .in time to approximately twice that of said predeterminedtime interval, said receiver including means for developing said firstand second output voltages during the fixed position time of saidradiation sensing means, analogue to digital conversion means forproviding said first and second output voltages in terms of bits versustime to said computer, said computer computing from said bits versustime input the slope and time axis intercept of a straight line fit tothe input data, said computer being programmed to compute from saidcomputed line slope and time axis intercept the ratio of the rate ofchange of angle of said source of radiated energy with respect to theknown rate of change of angular velocity of said energy radiatingsource, said computer being programmed to further translate calculatedpointing error signals into error in said computer first inputparameter, and said inertial measurement unit receiving said computerfirst input parameter error and including means to update said computerfirst input parameter in accordance with said error.

2. A navigation system as defined in claim 1 comprising means forsampling said receiver output voltages at a predetermined rate duringeach said calculation period, means for resetting said receiver outputvoltages to zero at the conclusion of each said calculation period andtiming means effecting successive ones of said calculation periods on apredetermined cyclic ba- SlS.

3. In a tracking system of the type designed to track an energyradiating source with known orbital parameters, means for pointing theboresight axis of a steerable radiation sensing means at a computedfuture position of said energy radiating source, said radiation sensingmeans developing orthogonal output voltages, means integrating theorthogonal output of said radiation sensing means, means for samplingthe levels of said means for integrating at a predetermined bit rate foran ensuing time period during which said energy radiating source passesthrough the aperture of the fixed radiation sensing means, meansresponsive to the output of said means for sampling for computing therespective orthogonal pointing errors of the boresight axis of saidradiation sensing means, said means for computing calculating the slopesand intercepts of straight lines fit to the output of said samplingmeans, which intercepts are proportional to respective orthogonalpointing errors.

4. In a tracking system of the type employing a positionable radiationsensing means, means for nutating said radiation sensing means about asensing means boresight axis, and positioning means for steering saidradiation sensing means to point said boresight axis at a radiationsource; a method of positioning said sensing means boresight axiscomprising the steps of; pointing the boresight axis at a predictedfuture location of said radiating source, maintaining said sensing meansat said predicted future location for a predetermined time interval assaid radiating source moves linearly through the region conicallyscanned by said sensing means about said boresight axis, sampling duringsaid predetermined time interval elevation and line-of-sight bearingchannel energy level outputs of said sensing means at a predeterminedsampling rate, deriving from said sampled outputs the slope and timeaxis intercept of a straight line fitted to said data using the methodof least squares, determining the boresight axis pointing error fromsaid slope and time axis intercept of said fitted line and the slope ofthe predicted line defined by radiating source ephemeris data,calculating from said pointing error, a subsequently assumed antennageographical position, and said radiation source ephemeris a commandsignal, pointing the sensing means boresight axis in response to saidcommand signal in the direction of a subsequent future radiation sourceposition, and repeating the above defined steps at a predeterminedcyclic rate.

5. A navigation system for a moving vehicle comprising a receiver ofenergy from an energy radiating source for which orbital parameters areto be determined, a computer means, said receiver including radiationsensing means and means for nutating said sensing means about asteerable boresight axis, said receiver developing first and secondoutput voltages respectively indicative of instantaneous bearing andelevation axis pointing errors between the boresight axis of saidsensing means and said energy radiating source, means for feeding systemgeographical position signals and assumed radiation source orbitalparameters as inputs to said computer, said computer being programmed tocalculate from said assumed radiation source ephemeris data and saidsystem geographical position signals bearing and elevation pointingcommands for application to said receiving means based on a computedfuture position of said energy radiating source a predetermined timeinterval from the instant of said calculation, said computer beingprogrammed to effect an ensuing calculation period during which zeropointing command is applied to said receiving means and during whichtime said energy radiating source passes through the aperture of saidradiation sensing means, said calculation period corresponding in timeto approximately twice that of said predetermined time interval, saidreceiver including means for developing said first and second outputvoltages during the fixed position time of said radiation sensing meansand for providing said first and second output voltages in terms of bitsversus time to said computer, said computer computing from said bitsversus time input the line slope and time axis intercept of a straightline fit to the bits versus time input, said computer being programmedto compute from said computed slope and time axis intercept parametersthe ratio of the rate of change of angle of said source of radiatedenergy with respect to the assumed rate of change of angular velocity ofsaid energy radiating source, said computer being programmed to furthertranslate calculated pointing error signals into error in the assumedradiation source orbital parameters initially provided said computer,and means to update the assumed radiation source orbital parameters asapplied to said computer in accordance with the error in the assumedvalues thereof supplied from said computer.

6. A navigation system as defined in claim comprising means for samplingsaid receiver output voltages at a predetermined bit rate during eachsaid calculation period, means for resetting said receiver outputvoltages to zero at the conclusion of each said calculation period, andtiming means effecting successive ones of said calculation periods on apredetermined cyclic ba- SIS.

7. In a tracking system of the type employing a positionable radiationsensing means, means for nutating said radiation sensing means about asensing means boresight axis, and positioning means for steering saidradiation sensing means to point said boresight axis at a radiatingsource the position of which is to be determined; a method ofpositioning said sensing means boresight axis comprising the steps of;pointing the boresight axis at a predicted future location of saidradiating source, maintaining said sensing means at said predictedfuture location for a predetermined time interval as said radiatingsource moves linearly through the region conically scanned by saidsensing means about said boresight axis, sampling during saidpredetermined time interval elevation and line-of-sight bearing channelenergy level outputs of said sensing means at a predetermined samplingrate, deriving from said sampled outputs the slope and time axisintercept of a straight line fitted to said data using the method ofleast squares, determining the boresight axis pointing error from slopeand time axis intercept of said fitted line and the slope of thepredicted line defined by radiating source assumed orbital parameters,calculating from said pointing error, a subsequently assumed radiatingsource position, and known sensing means geographical position a commandsignal; pointing the sensing means boresight axis in response to saidcommand signal in the direction of subsequent future radiating sourceposition, and repeating the abovedefined steps at a predetermined cyclicrate.

1. A navigation system comprising a receiver of energy from an energyradiating source having known orbital parameters, a computer means, aninertial measurement means, said receiver including radiation sensingmeans and means for nutating said sensing means about a steerableboresight axis, said receiver developing first and second outputvoltages respectively indicative of instantaneous bearing and elevationaxis pointing errors beTween the boresight axis of said sensing meansand said energy radiating source, said inertial measurement meansdeveloping output signals defining the earth geographical position ofsaid radiating sensing means, means for feeding said inertialmeasurement unit output signals as a first input parameter to saidcomputer, said computer receiving ephemeris data defining thetime-position of said energy radiating source as a second inputparameter thereto, said computer being programmed to calculate from saidfirst and second input parameters, bearing and elevation pointingcommands for application to said radiation sensing means based on acomputed future position of said source of energy radiation apredetermined time interval from the instant of said calculation, saidcomputer being programmed to effect an ensuing calculation period duringwhich zero pointing command is applied to said radiation sensing meansand during which time said energy radiating source passes through theaperture of said radiation sensing means, said calculation periodcorresponding in time to approximately twice that of said predeterminedtime interval, said receiver including means for developing said firstand second output voltages during the fixed position time of saidradiation sensing means, analogue to digital conversion means forproviding said first and second output voltages in terms of bits versustime to said computer, said computer computing from said bits versustime input the slope and time axis intercept of a straight line fit tothe input data, said computer being programmed to compute from saidcomputed line slope and time axis intercept the ratio of the rate ofchange of angle of said source of radiated energy with respect to theknown rate of change of angular velocity of said energy radiatingsource, said computer being programmed to further translate calculatedpointing error signals into error in said computer first inputparameter, and said inertial measurement unit receiving said computerfirst input parameter error and including means to update said computerfirst input parameter in accordance with said error.
 2. A navigationsystem as defined in claim 1 comprising means for sampling said receiveroutput voltages at a predetermined rate during each said calculationperiod, means for resetting said receiver output voltages to zero at theconclusion of each said calculation period and timing means effectingsuccessive ones of said calculation periods on a predetermined cyclicbasis.
 3. In a tracking system of the type designed to track an energyradiating source with known orbital parameters, means for pointing theboresight axis of a steerable radiation sensing means at a computedfuture position of said energy radiating source, said radiation sensingmeans developing orthogonal output voltages, means integrating theorthogonal output of said radiation sensing means, means for samplingthe levels of said means for integrating at a predetermined bit rate foran ensuing time period during which said energy radiating source passesthrough the aperture of the fixed radiation sensing means, meansresponsive to the output of said means for sampling for computing therespective orthogonal pointing errors of the boresight axis of saidradiation sensing means, said means for computing calculating the slopesand intercepts of straight lines fit to the output of said samplingmeans, which intercepts are proportional to respective orthogonalpointing errors.
 4. In a tracking system of the type employing apositionable radiation sensing means, means for nutating said radiationsensing means about a sensing means boresight axis, and positioningmeans for steering said radiation sensing means to point said boresightaxis at a radiation source; a method of positioning said sensing meansboresight axis comprising the steps of; pointing the boresight axis at apredicted future location of said radiating source, maintaining saidsensing means at said predicted future location for a predEtermined timeinterval as said radiating source moves linearly through the regionconically scanned by said sensing means about said boresight axis,sampling during said predetermined time interval elevation andline-of-sight bearing channel energy level outputs of said sensing meansat a predetermined sampling rate, deriving from said sampled outputs theslope and time axis intercept of a straight line fitted to said datausing the method of least squares, determining the boresight axispointing error from said slope and time axis intercept of said fittedline and the slope of the predicted line defined by radiating sourceephemeris data, calculating from said pointing error, a subsequentlyassumed antenna geographical position, and said radiation sourceephemeris a command signal, pointing the sensing means boresight axis inresponse to said command signal in the direction of a subsequent futureradiation source position, and repeating the above defined steps at apredetermined cyclic rate.
 5. A navigation system for a moving vehiclecomprising a receiver of energy from an energy radiating source forwhich orbital parameters are to be determined, a computer means, saidreceiver including radiation sensing means and means for nutating saidsensing means about a steerable boresight axis, said receiver developingfirst and second output voltages respectively indicative ofinstantaneous bearing and elevation axis pointing errors between theboresight axis of said sensing means and said energy radiating source,means for feeding system geographical position signals and assumedradiation source orbital parameters as inputs to said computer, saidcomputer being programmed to calculate from said assumed radiationsource ephemeris data and said system geographical position signalsbearing and elevation pointing commands for application to saidreceiving means based on a computed future position of said energyradiating source a predetermined time interval from the instant of saidcalculation, said computer being programmed to effect an ensuingcalculation period during which zero pointing command is applied to saidreceiving means and during which time said energy radiating sourcepasses through the aperture of said radiation sensing means, saidcalculation period corresponding in time to approximately twice that ofsaid predetermined time interval, said receiver including means fordeveloping said first and second output voltages during the fixedposition time of said radiation sensing means and for providing saidfirst and second output voltages in terms of bits versus time to saidcomputer, said computer computing from said bits versus time input theline slope and time axis intercept of a straight line fit to the bitsversus time input, said computer being programmed to compute from saidcomputed slope and time axis intercept parameters the ratio of the rateof change of angle of said source of radiated energy with respect to theassumed rate of change of angular velocity of said energy radiatingsource, said computer being programmed to further translate calculatedpointing error signals into error in the assumed radiation sourceorbital parameters initially provided said computer, and means to updatethe assumed radiation source orbital parameters as applied to saidcomputer in accordance with the error in the assumed values thereofsupplied from said computer.
 6. A navigation system as defined in claim5 comprising means for sampling said receiver output voltages at apredetermined bit rate during each said calculation period, means forresetting said receiver output voltages to zero at the conclusion ofeach said calculation period, and timing means effecting successive onesof said calculation periods on a predetermined cyclic basis.
 7. In atracking system of the type employing a positionable radiation sensingmeans, means for nutating said radiation sensing means about a sensingmeans boresight axis, and positioning means for steering said radiationsensing meaNs to point said boresight axis at a radiating source theposition of which is to be determined; a method of positioning saidsensing means boresight axis comprising the steps of; pointing theboresight axis at a predicted future location of said radiating source,maintaining said sensing means at said predicted future location for apredetermined time interval as said radiating source moves linearlythrough the region conically scanned by said sensing means about saidboresight axis, sampling during said predetermined time intervalelevation and line-of-sight bearing channel energy level outputs of saidsensing means at a predetermined sampling rate, deriving from saidsampled outputs the slope and time axis intercept of a straight linefitted to said data using the method of least squares, determining theboresight axis pointing error from slope and time axis intercept of saidfitted line and the slope of the predicted line defined by radiatingsource assumed orbital parameters, calculating from said pointing error,a subsequently assumed radiating source position, and known sensingmeans geographical position a command signal; pointing the sensing meansboresight axis in response to said command signal in the direction ofsubsequent future radiating source position, and repeating theabove-defined steps at a predetermined cyclic rate.